Forest lost 2-1 to the Seagulls, whose winner came in second-half stoppage time.
Nottingham Forest came within minutes of securing a credible draw against promotion-chasing Brighton & Hove Albion on Monday.
But Steve Sidwell's winner seconds into stoppage time left the managerless Reds 20th in the Championship - and staring at their worst finish at this level since the 2004-05 relegation campaign.
After three successive defeats, Forest were keen to restore some pride against the Seagulls and despite going a goal down to Lewis Dunk's header, they managed to do so and were good value for a draw following Dexter Blackstock's equaliser.
They might have won it on another day, with Henri Lansbury failing to make pay a counter attack before Sidwell's strike left the hosts without a win in six.
Lansbury, making only his second start since recovering from an ankle injury, was one of two Forest stars to take to Twitter after the game, posting: "Gutted ! support was unreal we go again Saturday."
Gutted ! support was unreal we go again Saturday— Henri Lansbury (@lansburyhenri) April 11, 2016
And teammate Danny Fox echoed those sentiments, tweeting: "Absolutely gutted about the result tonight. Thought we deserved more! Need to bounce back on Saturday!"
Blackstock's goal was only his third of the campaign but, remarkably, his second in front of the Sky cameras, following his equaliser at Wolves in December.
And the striker's teammate Jamie Paterson, who is currently on loan to Huddersfield Town, was quick to pick up on the impressive stat.
"@dexblackstock23 loves a goal on sky", the winger posted.